The present invention generally relates to information signal recording medium reproducing apparatuses, and more particularly to an information signal recording medium reproducing apparatus provided with a demodulating circuit for demodulating a frequency modulated composite information signal which is reproduced from an information signal recording medium.
A rotary recording medium (hereinafter simply referred to as a disc) which is recorded with a frequency modulated composite video signal as variations (existence and non-existence of pits) in geometrical configuration, which frequency modulated composite video signal is obtained by frequency-modulating a carrier by a composite video signal, and a reproducing apparatus for reproducing the recorded frequency modulated composite video signal from the disc, have been developed and reduced to practice, and the disc and the reproducing apparatus are put into wide and general use. The reproducing apparatus scans the disc by an electrostatic capacitance type stylus (or laser beam), and picks up the recorded frequency modulated composite video signal from the disc responsive to variations in the electrostatic capacitance between the stylus and the disc, which variations in the electrostatic capacitance are in accordance with the variations in the geometrical configuration of the disc. The frequency modulated composite video signal which is picked up, is frequency-demodulated into a reproduced composite video signal. The disc is also recorded with an audio signal, and the signals which are recorded on the disc will hereinafter be generally referred to as an information signal.
The frequency modulated composite video signal which is recorded on the disc of the type described above, is a signal which has been subjected to such a frequency modulation that a white peak of the composite video signal corresponds to an upper limit (for example, 7.9 MHz) of the frequency deviation and the tip end of the horizontal synchronizing signal (so-called sync tip) corresponds to a lower limit (for example, 6.1 MHz) of the frequency deviation. Accordingly, the interval of the pits which are formed on the disc in accordance with the frequency modulated composite video signal, is narrow at a recorded part corresponding to the white level of the composite video signal and is wide at a recorded part corresponding to the horizontal synchronizing signal of the composite video signal.
However, in the case where scratches and the like exist on a stamper at the time when the disc is molded, or in the case where foreign particles and the like are mixed into a material from which the disc is molded, unwanted pits (false pits) are formed on the disc in addition to the pits of the recorded signal. When such false pits exist on the disc, the frequency modulated signal is picked up as a signal having a higher frequency at parts on the disc where the false pits exist. Especially in the case where the false pits exist on the disc at a recorded part corresponding to a horizontal synchronizing signal, the part of the frequency modulated signal corresponding to the horizontal synchronizing signal is picked up as a signal having a high frequency, and the waveform of the demodulated horizontal synchronizing signal becomes distorted when the picked up signal is demodulated.
On the other hand, the horizontal synchronizing signal within the composite video signal which is picked up from the disc and demodulated (reproduced), is separated from the reproduced composite video signal and is used for various purposes such as obtaining a control signal for carrying out jitter compensation and control of a reproducing element of the reproducing apparatus and obtaining a control signal for an automatic frequency control (AFC) circuit of a television receiver. Especially the front edge of the horizontal synchronizing signal is used when obtaining such control signals. However, when the front edge part of the horizontal synchronizing signal is missing or the waveform at the front edge part of the horizontal synchronizing signal is distorted due to the false pits described before, the above control signals cannot be obtained properly. As a result, there are problems in that it becomes impossible to carry out a satisfactory jitter compensation and the receiver becomes out of horizontal synchronism. Hence, it is extremely important that the horizontal synchronizing signal within the reproduced composite video signal, especially the front edge part of the horizontal synchronizing signal, is not missing nor distorted.